OpenGL.org Web site; lots of information on OpenGL in general; specs for OpenGL 2.0 and GLSL.
Shadertech Real-time shader development Web site.
RenderMonkey An IDE supporting several modes of shader development.
Shader Designer Another IDE for writting GLSL shaders.
Book web sites: OpenGL Shader Language, GPU Gems, GPU Gems 2. Also see Shadertech.